qtnfmac: retrieve current channel info from EP

Do not try to cache current operational channel info in driver, this
is a potential source of synchronization issues + driver does not
really need that info.

Introduce GET_CHANNEL command and process it appropriately.

Signed-off-by: Igor Mitsyanko <igor.mitsyanko.os@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>